A prior art fire detector comprises a sensor which outputs a voltage corresponding to smoke concentration, temperature, or the like and a comparator which compares the sensor output and a reference voltage. It produces a fire signal according to the result of the comparison, i.e., the difference between the two voltages compared. The reference voltage is set to a sensor output level corresponding to a certain smoke level, for instance, according to the type of the sensor. This setting is, of course, carried out manually. Usually the reference voltage is variable and can be set to a suitable value. Once it is set, however, it constitutes a criterion for determining whether there is a fire. Therefore, a fire signal is produced whenever the sensor output exceeds the reference voltage even when there is actually no fire. False alarms do in fact often occur with a fixed reference voltage depending upon the environment of the site of installation of the fire detector, the sensitivity fluctuations thereof and other factors. With the prior art fire detector, however, it is difficult to take effective measures to change the preset reference voltage when false alarms occur. Therefore, once a false alarm occurs, it will occur again if the same conditions are met. In some other prior art fire detectors, the time differential or integral of the sensor output is, instead of the sensor output, compared with the reference voltage. Even in this case, the drawback as discussed above cannot be avoided.
The object of the invention is to provide a fire detecting system, which overcomes the above drawback inherent in the prior art, and which permits the preset reference voltage to be changed when a false alarm occurs, thus preventing a false fire signal from being produced when the same conditions are subsequently met.